A monster avalanche stormed down Mt Ruapehu earlier this week after it was triggered by avalanche control.
Dangerous snow buildup at Tūroa forced staff at Mt Ruapehu's Whakapapa and Tūroa ski fields to use explosives in areas identified by the snow safety team as areas of weakness.
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This created a "significant" avalanche, which was followed by a second one hours later, closing Tūroa. Video of one of the triggered avalanches shows the massive wall of snow tearing downhill.
Mt Ruapehu general manager safety and environment Andy Hoyle says they show the importance of avalanche control work.
"As part of our process of looking after our people, customers and resources, we are duty and morally bound to actively manage these hazards as soon as we practicably can.
"We know that if we left these avalanche hazards uncontrolled, they could occur naturally in the right conditions, threatening personnel and infrastructure."
The ski field says making sure the mountain is safe is a science, with skills acquired over years and years of working in the mountains. The snow safety officers study the buildup of snow deposits and layers, searching for danger zones.
"Storms bring precipitation in the form of snow, hail, sleet and wind. Each storm leaves its mark in the stratigraphy of the snowpack," Hoyle says.
"Layers build upon layers, a kind of temporary record of time. Oldest at the bottom, youngest on top. Occasionally, some layers are weaker than others and can fail when the load of the layers above gets too heavy. Once things start to move, this is known as an avalanche."